Secondary batteries are representative electrochemical devices in which external electrical energy converted into chemical energy is stored and created into electricity when necessary. Secondary batteries are also called “rechargeable batteries,” because they are capable of repeated charge and discharge. Lead-acid batteries, nickel cadmium (NiCd) batteries, nickel metal hydride (NiMH) batteries, lithium ion batteries and lithium ion polymer batteries are frequently used as secondary batteries. Secondary batteries offer economic and environmental advantages over primary batteries that are disposed of after energy stored therein has been exhausted.
Secondary batteries are currently used in applications where low power is needed, for example, devices for assisting in the starting of car engines, portable devices, instruments and uninterrupted power supply systems. Recent developments in wireless communication technologies have led to the popularization of portable devices and have brought about a tendency to connect many kinds of existing devices to wireless networks. Under such circumstances, demand for secondary batteries is growing explosively. Hybrid automobiles and electric automobiles have been put into practical use for the purpose of preventing environmental pollution. These next-generation automobiles reduce costs and weight and increase their life span by employing technologies based on secondary batteries.
Generally, most secondary batteries are cylindrical, prismatic or pouch type in shape depending on the fabrication process thereof. That is, a secondary battery is typically fabricated by inserting an electrode assembly composed of an anode, a cathode and a separator into a cylindrical or prismatic metal can or a pouch-type case made of an aluminum laminate sheet, and injecting an electrolyte into the electrode assembly. Accordingly, the cylindrical, prismatic or pouch-type secondary battery requires a certain space for assembly, which is an obstacle to the development of various types of portable devices. Thus, there is a need for a novel type of secondary battery whose shape is easy to change, and particularly, an electrolyte suitable for use in the secondary battery that has high ionic conductivity without any risk of leakage.
Ionically conductive organic electrolytes in the form of liquids in which salts are dissolved in non-aqueous organic solvents have predominantly been used in conventional electrochemical devices based on electrochemical reactions. However, the use of such electrolytes in the form of liquids causes degradation of electrode materials, increases the possibility of evaporation of organic solvents, and poses safety problems, such as fire resulting from high surrounding temperatures and increased battery temperatures. There are other problems, such as a risk of leakage and a difficulty in realizing various types of electrochemical devices. In attempts to overcome the safety problems of such liquid electrolytes, polymer electrolytes have been proposed, such as gel polymer electrolytes and solid polymer electrolytes. It is generally known that the safety of electrochemical devices increases in the order of liquid electrolytes, gel polymer electrolytes and solid polymer electrolytes, but the performance thereof decreases in the same order. Electrochemical devices employing solid polymer electrolytes are not yet commercialized, to our knowledge, because of their inferior performance. Gel polymer electrolytes have low ionic conductivity, suffer from the risk of leakage and possess poor mechanical properties compared to liquid electrolytes.
Korean Unexamined Patent Publication No. 2008-33421 discloses an electrolyte using a plastic crystal matrix instead of a liquid organic solvent. The electrolyte exhibits ionic conductivity comparable to that of a liquid electrolyte. However, the electrolyte exhibits very poor mechanical properties due to its flowability similar to that of liquid. In actuality, a separator is required to prevent short circuits in a battery using the electrolyte. In some cases, the introduction of linear polymer matrices, such as polyethylene oxide, is considered to improve the mechanical strength of plastic crystal matrix electrolytes. In these cases as well, the electrolytes do not possess mechanical properties sufficient to negate the need for separators. Drying is necessary to remove solvents used to dissolve the linear polymers, rendering the process complicated.
Thus, there is an urgent need to develop a solid electrolyte using a plastic crystal matrix electrolyte that has improved mechanical properties while maintaining high ionic conductivity of the plastic crystal matrix electrolyte.